1. Field of the Invention
The present invention relates generally to tuning phase-change resistive elements. More particularly, the present invention relates to a method and apparatus for using an off-chip precision resistor as a reference to tune one or more on-chip phase-change resistive elements.
2. Description of the Related Art
The electrical resistance of resistance switching materials (i.e., phase-change materials), including but not limited to chalcogenides and alloys thereof, and perovskites or perovskite-like materials (e.g., colossal magnetorisistance (CMR) materials and high temperature superconductivity (HTSC) materials), can be changed significantly by external influences, including temperature, magnetic fields and electric fields. In their amorphous states, these phase-change materials exhibit a higher resistance because electrons move slower through the material. In semi-crystalline or semi-amorphous states, resistance switching materials can exhibit less resistance. Finally, in their crystalline states, resistance switching materials exhibit low resistance because electrons are able to move fast through the material. Electrical impulses applied to theses materials can “tune” or “program” them, such that they exhibit the desired resistive property. For example, applying a “set” current having a specific pulse height and width to a phase-change material will crystallize it, thereby, decreasing the resistance. Similarly, applying a “reset” current having another specific pulse height and width to the phase change material will return the material to its amorphous state, thereby, increasing the resistance. The following U.S. Patents, and U.S. Patent Applications, incorporated herein by reference, discuss phase-change materials and methods for switching the resistances of such materials: U.S. Pat. No. 6,673,691 issued to Zhuang et al. on Jan. 6, 2004; U.S. Pat. No. 6,204,139 issued to Liu et al. on Mar. 20, 2001; and, U.S. Patent Application Pub. No. US 2004/0252544 to Lowery et al. on Dec. 16, 2004. Incorporating phase-change resistive elements into integrated circuits is known. For example, phase-change resistive elements can be used to form programmable resistance memory elements, as disclosed in the following U.S. Patents and U.S. Patent Application, incorporated herein by reference: U.S. Patent Application Pub. No. US 2004/0252544, referred to above, U.S. Pat. No. 6,608,773 issued to Lowrey et al. on Aug. 16, 2003; and, U.S. Pat. No. 6,795,338 issued to Parkinson et al. on Sep. 21, 2004. Similarly, U.S. Pat. No. 5,296,835, issued to Nakamura on Mar. 22, 1994 (incorporated herein by reference) disclosed a device that incorporates a variable resistor constructed to a desirable resistance using a chalcogenide semiconductor. However, precision tuning of on-chip phase-change resistive elements to a target resistance with a tight tolerance can be difficult. The present invention relates to a method and apparatus for the precision tuning of one or more on-chip phase-change resistive elements.